1. Field of the Invention
The present invention relates to a method for forming a prearranged rupture portion for an air bag door in a vehicle interior member composed of a substrate and layered materials that cover a surface of the substrate and more particularly to a method for forming a prearranged rupture portion for an air bag door in a vehicle interior member that has a prearranged rupture portion defined by grooves formed on a back surface of the vehicle interior member.
2. Background Art
As shown in FIG. 5, an air bag device 14 is generally disposed on the backside of an instrument panel that is in front of the passenger seat of a vehicle. More specifically, an air bag door 12 of the air bag device 14 is provided on an instrument panel 10, and, when the air bag device 14 is activated, the air bag door 12 is ruptured along the prearranged rupture portion 16 to be opened, allowing an air bag 15 to be inflated and blown out toward the cabin of the vehicle. The prearranged rupture potion 16 of the air bag door 12 is a portion which is weakened by providing a groove 18 with a predetermined depth on the back surface of a substrate 20 that constitutes the instrument panel 10. Accordingly, the prearranged rupture portion 16 does not appear nor visible on the surface of the instrument panel 10.
The prearranged rupture portion 16 is required to be ruptured quickly and surely when the air bag device 14 is activated. On the other hand, the air bag door 12, in a state before the air bag device 14 is activated, forms a part of the instrument panel 10, and the prearranged rupture portion 16 is required to have a structural strength of such an extent that it does not cause unintended rupture or deformation. The ease of rupture and strength of the prearranged rupture portion 16 are defined by the size of thickness of the substrate 20 after the formation of the groove 18, i.e., the thickness of the rest of the substrate (hereinafter, referred to as “remaining thickness”). Further, in order to keep the balance between the ease of rupture and the strength of the prearranged rupture portion 16, it is required to precisely manage the remaining thickness of the substrate.
Examples of the method of forming the prearranged rupture portion 16 include a method of cutting to form a groove 18 with a predetermined depth from the back surface side of the substrate 20 after the molding of the substrate 20 into a desired shape. In this method, the prearranged rupture portion 16 is formed by cutting to form the groove 18 with a predetermined depth in the substrate 20 placed and fixed on a fixing jig, while moving a cutting blade such as an end mill in a predetermined direction. In this case, the remaining thickness is managed by controlling the position of the cutting blade relative to the set surface of the fixing jig by way of referring to positional data adapted to the set surface of the fixing jig formed so as to conform in to the shape of the surface of the substrate 20.
Generally, the surface of the substrate 20 of the instrument panel 10 is, as seen from FIG. 5, covered by a foamed layer 22 and a surface skin 24 (see, e.g., Japanese Patent Application Laid-Open (Kokai) No. 2008-290643). The prearranged rupture portion 16 of the air bag door 12 of this art is constituted from the groove 18, which is not perforating the substrate 20, and a groove (not shown), which is perforating through the substrate 20 and the foamed layer 22 to reach the surface skin 24. Since the surface skin 24 is generally softer than the substrate 20, the influence of the groove is likely to appear on the surface of the surface skin 24. Thus, the quality of the appearance is largely affected by the remaining thickness of the surface skin. In other words, with respect to the prearranged rupture portion 16 of the air bag door 12 disclosed in Japanese Patent Application Laid-Open (Kokai) No. 2008-290643, it is necessary to precisely manage each of the remaining thickness of the substrate and the remaining thickness of the surface skin.
The instrument panel 10 in a layered structure as described above has a disadvantage that the thickness of the foamed layer 22 is likely to vary compared with the thickness of the substrate 20. Accordingly, when the foamed layer 22 and the surface skin 24 are formed into a layered structure, errors in the thickness can accumulate, causing the precision of the thickness to be lowered compared with an instrument panel in which the substrate 20 has a single-layered structure. Further, in a method of cutting to form the groove 18 only in the substrate 20 with reference to the set surface of a fixing jig, if the sizes of thickness of the foamed layer 22 and the surface skin 24 are smaller than the designed sizes, then the remaining thickness of the substrate becomes larger. In addition, if the sizes of thickness of the foamed layer 22 and the surface skin are larger than the designed sizes then the remaining thickness of the substrate becomes smaller. Accordingly, it is not possible to precisely manage the remaining thickness of the substrate.